Russian warplanes bombed a Syrian Arab Coalition force trained and advised by U.S. troops in northeastern Syria on Tuesday and inflicted casualties, the top U.S. commander for Iraq and Syria said Wednesday.

U.S. troops working with the coalition were within three miles of the airstrikes and were not endangered, said Army Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force-Operation Inherent Resolve.

The U.S. troops observed the airstrikes and notified higher command to contact the Russians under an arrangement to "deconflict" U.S. and Russian operations in Syria, he said in a briefing from Baghdad to the Pentagon.

The Russians quickly called off the airstrikes, said Townsend, who suggested that the bombings were likely a mistake in what he called a "highly complicated battlefield" in northeastern Syria.

Townsend said the Syrian Arab Coalition fighters suffered casualties but declined to say how many or whether there were fatalities. He said the U.S. leaves it to partnered forces to report on casualties they may suffer.

The general said the Russian airstrikes occurred in an area about 10-12 miles west of Manbij, which was liberated from the Islamic State of Iraq and Syria last year by a combined Syrian Kurdish and Syrian Arab force supported by the U.S. with airstrikes, supplies, training and advice.

The area has seen a confluence of forces in recent weeks competing for influence and control.

Forces of the Syrian regime of President Bashar al-Assad, backed by Russian airstrikes, have moved into the region from the south. Elements of the Turkish military supporting the Free Syrian Army militia are also operating against ISIS in the area. In addition, there is the Syrian Arab Coalition backed by the U.S.

"It's a very complicated battlefield situation, where essentially three armies and an enemy force have converged within the same grid square," Townsend said. "It's a very difficult and complicated area" in which to operate.